Precious metals are used as protective films on surfaces for a variety of reasons. In the jewelry trade, it is used to improve the appearance of an article as in gold plated jewelry. In other applications, it is used to protect against corrosion of metals and other surface materials. In the electrical arts, protective films made of precious metals are used as conduction paths in electrical circuits and as contact surfaces in devices with electrical contacts. Gold is used extensively in these applications with great success. However, the increased price of gold makes it attractive to look at other precious metals as protective films on various surfaces.
Silver and silver alloys are used extensively in a variety of industrial applications. Typical examples are the jewelry trade where such films are used to protect surfaces against corrosion and to improve appearance and the electrical arts in various electrical devices and electronic circuits. Silver is used as conducting paths in various types of printed circuits and integrated circuits.
Because of relative chemical inertness and reasonable hardness, silver is especially attractive as an electrical contact material in electrical connectors, relay contacts, switches, etc. Indeed, because of the increasing cost of gold, silver and silver alloys become more and more attractive economically as a contact material, surface material and in other applications. In many applications where gold is now used, it is often economically attractive to use silver, provided an inexpensive and efficient method of plating ductile and adherent silver is available.
Silver is also used as a brazing material and is sometimes applied by electroplating in the manufacturing of devices. Examples are planar triodes and surge protectors.
Highly desirable is a process for plating silver and silver alloys from an aqueous solution which is operable at high rates of deposition and yields silver and silver-alloy films which are ductile and adherent.
Conventionally, silver is electroplated from a cyanide-type bath in basic solution. Such processes have been described in a number of references, including: U.S. Pat. No. 2,777,810, issued to B. D. Ostrow on Jan. 5, 1957; U.S. Pat. No. 2,735,808, issued to Lawrence Greenspan on Feb. 21, 1956 and U.S. Pat. No. 2,666,738, issued to Otto Kardos on Jan. 19, 1954. Although such processes yield satisfactory results from a pollution, disposal and environmental point of view, it is often desirable to avoid cyanide baths. Also, replenishment of the baths often involves introduction of additional anions already present in the electroplating bath or different anions not already present in the bath. The introduction of such ions often complicates the silver electroplating process and often, because of buildup of the anion concentration, limits the lifetime of the electroplating bath. A cyanide-free silver electroplating process where anion concentration remains relatively constant is highly desirable.
K. Hosokawa et al in a paper published in Proceedings of the 8th Congress of the International Union for Electrodeposition and Surface Finishing, Forster-Verlag AG, Zurich, 1973, pp. 180-186, described some experiments in which silver is plated from an aqueous solution containing silver-ethylenediamine complex ion. Although the paper contains much discussion on the mechanism for electroplating from such a solution, the commercial value of the process is not apparent.